The present invention relates to fiber optic transmission and more particularly to C-band and L-band amplifiers used for submarine transmission systems.
Fiber optic transmission systems use various bands, especially terrestrial transmission systems. The expression xe2x80x9cC bandxe2x80x9d refers to the range of wavelengths from 1529 to 1565 nm and the term xe2x80x9cL bandxe2x80x9d refers to the range of wavelengths from 1569 to 1603 nm. More generally, different values, beyond 1565 nm, can be considered for the L band, over a bandwidth of the order of 30 or 35 nm.
M. X. Ma et al, xe2x80x9c765 Gbyte/s over 2,000 km Transmission using C- and L-band Erbium Doped Fiber Amplifiersxe2x80x9d, OFC""99, post-deadline paper PD16-1 to 3 describes a transmission experiment in the C and L bands for submarine transmission systems with 43.5 km sections and optical amplifiers. Each amplifier consists of a band splitter, separate amplification branches for the C and L bands, and a band combiner.
Y. Sun et al, xe2x80x9cA gain-flattened ultra wide band EDFA for high capacity WDM optical communications systemsxe2x80x9d, ECOC""98, pp. 53-54 proposes, for a terrestrial fiber optic transmission system, a very wide band optical amplifier for amplifying signals in the C and L bands. The amplifier has a first amplification stage with a fiber in which the C-band and L-band signals are amplified conjointly by contrapropagating light at 980 nm. The C-band and L-band signals are then split and amplified separately in two separate branches; each branch has three amplification stages to compensate the effects of the dispersion compensating fiber (DCF), to compensate the losses of the gain equalization filters, and to amplify the signals to the output power. For an input power of 0 dBm, the total gain of the amplifier is 25 dB. Approximately a quarter of the power created in the C band in the amplifier is created in the first amplification stage and three quarters are created in the branch for the C-band signals of the second stage.
The subject matter of the invention is amplification of C-band and L-band signals for submarine transmission systems; in such systems the amplifier input power is relatively high but the gain is moderatexe2x80x94the article previously cited by M. X. Ma proposes a value of 7 dBm for the input power and a value of 11.7 dB for the gain.
To be more precise, the invention proposes an optical amplifier for the C and L bands, having a first stage with an amplification fiber common to the C and L bands, a second stage with a branch for C-band signals and a branch for L-band signals with an amplification fiber, in which most of the power created in the C band is created in the first amplification stage.
The ratio between the output power in mW of the first stage for C-band signals and the output power mW of the amplifier for C-band signals is preferably greater than 0.50 and preferably greater than 0.75.
The gain of the amplifier for C-band signals is advantageously equal to the amplification gain for C-band signals in the first amplification stage.
The amplifier preferably has a gain less than 15 dB.
In one embodiment the amplifier has a coupler in the branch for C-band signals coupling pumping light for the amplification fiber of the first stage in the contrapropagating direction. The pumping light can have a wavelength from 1450 to 1500 nm.
There can be a coupler in the first stage for coupling second pumping light for the amplification fiber of the first stage. The second pumping light can have a wavelength from 965 to 990 nm.
Another embodiment of the amplifier has a coupler in the branch for L-band signals coupling pumping light for the amplification fiber of said branch.
In this case, it advantageously has a pump for supplying pumping light to the coupler in the branch for C-band signals and to the coupler in the branch for L-band signals.
The invention also relates to a fiber optic transmission system including at least one such amplifier.